1. Field of the Invention
The invention relates to an antenna, more particularly to an antenna that is provided with a diffraction grating modulator to improve antenna gain and side lobe bandwidth and to modulate directionality of the antenna.
2. Description of the Related Art
Antennas are used in the communications field as a system for sending and receiving signals, and are applied for signal reception in household appliances, such as television sets and radios, for signal transmission in pagers and cellular telephones, and in satellite communication systems. With the advent of personal communications, the use of most conventional antennas, such as Yagi-Uda antennas, whip antennas and disk antennas, has become impractical in electronic communication devices in view of the growing trend toward miniaturization of these devices.
Different types of antennas are available in the market. Each type of antenna has its particular characteristics. For example, a conventional Yagi-Uda antenna includes a driven element, a reflector behind the driven element, and one or more directors in front of the driven element. While the lengths of the reflector and the director, and the spacing of each of the reflector and director with the driven element, can be adjusted for optimum directivity and high gain, the Yagi-Uda antenna is too big for use in personal electronic communication devices.
Some of the demands of a modular communications system include lightweight, portability, clear reception, and modular design. By increasing antenna mobility, the design, manufacture and maintenance of the entire communications system can be facilitated. Microstrip antennas and slot antennas are among the few known antennas that can satisfy the above demands to result in a good communications system design.
The microstrip antenna and the slot antenna have a simple construction and can be attached to the surface of any object without affecting the appearance of the latter. Furthermore, these antennas can be formed directly with a circuit component on a monocrystal microwave integrated circuit (MMIC). As such, the microstrip antenna and the slot antenna are widely used in personal mobile communication systems.
Referring to FIG. 1, a conventional microstrip antenna 2 is shown to include a substrate 21 disposed on a ground plane, a microstrip patch 22 disposed on top of the substrate 21, and a signal feed line 24 with a first end connected to the microstrip patch 22 and a second end connected to the ground plane. When a signal between the substrate 21 and the microstrip patch 22 satisfies electromagnetic field resonance conditions, the microstrip patch 22 radiates and receives electromagnetic waves, thereby achieving the object of signal exchange. The radiation pattern of the conventional microstrip antenna 2 is shown in FIG. 2.
While the conventional microstrip antenna 2 offers numerous advantages, it suffers from the drawbacks of a narrow bandwidth of only about 1 to 2 percent, and low antenna gain, thereby seriously limiting the applicability of the microstrip antenna 2. Present efforts at overcoming the aforementioned drawbacks have been aimed at the development of array microstrip antennas.
In addition, the use of frequencies below the Ka frequency band for satellite communications has now become saturated, and there is an urgent need to develop a new frequency band for communications purposes. The Ka frequency band is a higher frequency band in the microwave frequency range, and ranges from 18 GHz to 40 GHz. Aside from being adapted for use in satellite communications, array microstrip antennas usable in the Ka frequency band can also be applied in inter-building data transmission to obviate the need for installing electrical or fiber optic cables. Moreover, because the frequencies that are in use fall within the Ka frequency band, interference with present microwave communication systems can be avoided.
In view of the foregoing, it can be understood that most conventional antennas suffer from one or more of the following drawbacks:
1. A high antenna gain usually involves a bigger antenna size.
2. In a conventional microstrip antenna, the bandwidth is too narrow, and the gain is too small.
3. Once the conventional antenna is mounted, the directionality thereof cannot be modulated.
4. The conventional antenna does not allow for a modular design. As such, a significant adjustment in the operating bandwidth is not possible for a single conventional antenna.